1. Field of the Invention.
This invention relates to electronic filters and, more particularly, to electronically variable equalizer filters having a variable cutoff frequency.
2. Prior Art.
Equalizer filters are used to shape the amplitude and phase response characteristics of a particular channel prior to processing of the signals carried by that channel. In many applications it is desirable that certain ones of the regular characteristics of the equalizer be variable. For example, it may be desirable to vary the bandwidth of an amplifier to accommodate certain operating conditions. Additionally, it may be desirable to change the gain characteristics by inserting, as required, selective boost, or gain, for certain bands of frequencies. For many applications involving digital or pulse signals, it is desirable that a constant group delay characteristic through the pass band be maintained. This ensures that all of the frequency components of a signal each have the same amount of time delay as they pass through the equalizer circuit so that the signal, or pulse, is properly amplified by the equalizer circuit without distortion in the absence of non-linear phase delay for the various frequency components.
Equalizers find application in a number of types of electronic equipment, including both conventional and constant density disk drives, magnetic tape recorders, wire recorders, optical recording devices, telecommunication channels, test equipment, radio, sonar, radar, control systems, satellite links, and other applications where amplitude, phase, and bandwidth shaping are required. These equipments and applications may require different transfer characteristics under various operating conditions and loads. For example, when it is known that an incoming signal has a restricted narrower bandwidth, it is desirable to narrow the pass band of the equalizer to improve the signal-to-noise ratio.
For example, in reading the information from a constant density magnetic disk which records a constant wavelength in the magnetic media disk, the servo signal clock frequency, or recovered read clock frequency, varies as a function of the physical position of a recording track on a disk. Assuming a constant rotational speed of the disk, the signal wavelength typically varies two-to-one from the inner to outer tracks. The information signals associated with the servo signal clock frequency will vary over a two-to-one range.
To optimize the signal-to-noise ratio for the information signals in a read channel, the bandwidth of an equalizer filter may be varied as a function of the servo signal clock frequency.
Prior art constant density magnetic recording systems change bandwidth, for example, by band-switching components to realize three or four discrete filter configurations.
Prior art approaches to providing a variation in characteristics for an equalizer include band switching transfer using, for example, analog switches to switch inductions and capacitors in three or four bands to obtain a desired performance characteristic.